Move IPC reception handling from AutofillManager to AutofillDriverImpl.
As AutofillManager will be shared by iOS, it should not receive IPC directly.
Instead, AutofillDriverImpl now handles IPC reception and forwards to
AutofillManager as appropriate.
Similarly, this CL also abstracts the content-level concepts of handling frame
navigation out of AutofillManager into AutofillDriverImpl.
BUG=247015
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=207726

LGTM.
It would be great to update the documentation for all of these methods, and
group them a little more sanely in the header, e.g. separating the
Autocheckout-related callbacks from the others. But, maybe that's better done
in a separate CL.